1. Field of the Invention
The present invention generally relates to nickel-base alloys. More particularly, this invention relates to castable and weldable nickel-base alloys exhibiting desirable properties suitable for gas turbine engine applications.
2. Description of the Related Art
The superalloy GTD-222 (U.S. Pat. No. 4,810,467) has a number of desirable properties for gas turbine engine applications, such as nozzles (vanes) in the latter (second and third) stages of the turbine section. The nominal composition of GTD-222 is, by weight, about 19% cobalt, about 22.5% chromium, about 2% tungsten, about 1.2% aluminum, about 2.3% titanium, Al+Ti of about 3.5%, about 0.8% columbium (niobium), about 1.0% tantalum, about 0.01% boron, about 0.01% zirconium, about 0.1% carbon, with the balance essentially nickel and incidental impurities. As with the formulation of other nickel-base alloys, the development of GTD-222 involved careful and controlled adjustments of the concentrations of certain critical alloying elements to achieve a desired mix of properties. For use in turbine nozzle applications, and particularly the latter stage nozzle for which GTD-222 is used, such properties include high temperature strength, castability, weldability, and resistant to low cycle fatigue, corrosion and oxidation. The thermal environment within the second stage of a turbine section is sufficiently severe to require an oxidation-resistant coating, a thermal barrier coating (TBC), and/or internal cooling for nozzles formed of the GTD-222 alloy. The properties of GTD-222 are sufficient to allow third stage nozzles to achieve the design life required of the nozzles without such additional measures
When attempting to optimize any one of the desired properties of a superalloy, other properties are often adversely affected. A particular example is weldability and creep resistance, both of which are of great importance for gas turbine engine nozzles. However, greater creep resistance results in an alloy that is more difficult to weld, which is necessary to allow for repairs by welding. A desirable combination of creep strength and weldability exhibited by GTD-222 is believed to be the result of the use of judicious levels of aluminum, titanium, tantalum and columbium in the alloy. Each of these elements participates in the gamma prime (xcex3xe2x80x2) precipitation-strengthening phase (Ni3(Ti,Al)). Aluminum and titanium are the key elements in the formation of the gamma-prime phase, while the primary role of tantalum and columbium is to participate in the MC carbide phase. Tantalum and columbium remaining after MC carbide formation plays a lesser but not insignificant role in the formation of the gamma-prime phase.
While GTD-222 has been proven to perform well as the alloy for latter stage nozzles of gas turbine engines, alternatives would be desirable. Of current interest is the reduction in tantalum used in view of its high cost. However, the properties of an alloy with a reduced tantalum content would preferably be closely matched with those of GTD-222, particularly for use as the alloy for second and third stage nozzles.
The present invention provides a nickel-base alloy that exhibits a desirable balance of strength (including creep resistance) and resistance to corrosion and oxidation suitable for nozzles of the latter stages of a gas turbine engine, particularly the second and third stage nozzles. The alloy is also castable, relatively easier to weld than GTD-222, and has acceptable heat treatment requirements. These desirable properties are achieved with an alloy in which tantalum is eliminated or at a relatively low level, and a relatively high level of columbium is maintained to achieve properties similar to that of the GTD-222 alloy.
According to the invention, the nickel-base alloy consists essentially of, by weight, 10% to 25% cobalt, 20% to 28% chromium, 1% to 3% tungsten, 0.5% to 1.5% aluminum, 1.5% to 2.8% titanium, 0.8% to 1.45% columbium, tantalum in an amount less than columbium and Cb+0.508Ta is 1.15% to 1.45%, 0.001% to 0.025% boron, up to 0.05% zirconium, 0.02% to 0.15% carbon, with the balance essentially nickel and incidental impurities. The columbium content of the alloy is preferably at least 0.9%, more preferably at least 1.25%, while the tantalum content of the alloy is preferably less than 0.5%, more preferably entirely omitted from the alloy.
The alloy of this invention has properties comparable to those of the GTD-222 alloy, with potentially improved ductility and weldability and with no degradation in castability. Notably, improved weldability of the alloy is achieved without sacrificing creep resistance. These properties and advantages are achieved even though the relative amounts of tantalum and columbium are opposite those of GTD-222, namely, more columbium is present in the alloy than tantalum, with a preferred maximum level of tantalum being below the minimum amount of tantalum required for GTD-222. The desired properties are believed to be achieved by maintaining a substantially constant combined atomic percent of columbium and tantalum in the alloy, in which columbium contributes greater to the combined amount than does tantalum as a result of specifying the combined amount according to the formula Cb+0.508Ta. Contrary to GTD-222 (U.S. Pat. No. 4,810,467), second and third stage nozzles exhibit excellent properties when cast from the alloy in which tantalum is essentially absent, i.e., only impurity levels are present. Consequently, the alloy of this invention provides an excellent and potentially lower-cost alternative to GTD-222 as a result of reducing or eliminating the requirement for tantalum.
Other objects and advantages of this invention will be better appreciated from the following detailed description.